Fixes devstack settings to work with new version of debug toolbar

[davestgermain]

@singingwolfboy
Your recent PR to upgrade the debug toolbar broke devstack because the SHOW_TOOLBAR_CALLBACK setting now expects a string rather than a function.

[flowerhack]

Does this fix work locally for you? I'm having the issue you described, but when I cherry-picked this commit I got the following error when I try to load the LMS:

Traceback (most recent call last):
File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 89, in get_response
response = middleware_method(request)
File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/debug_toolbar/middleware.py", line 46, in process_request
show_toolbar = getattr(import_module(mod_path), func_name)
AttributeError: 'module' object has no attribute 'should_show_debug_toolbar'

[davestgermain]

yeah, I was a bit hasty. did you cherry pick 5877e84? that should work

[flowerhack]

Ah, yes, the new commit works :) Thanks so much for getting this!

⛵ as soon as tests pass

[gwprice]

Revert this

[davestgermain]

OK, I added to cms settings as well.

[gwprice]

👍